题目链接

https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/

方法1 BFS

image.png

需要主要的是 结果有顺序要求 从下至上 从左至右

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:

        if not root:
            return []
        nodes=[root]

        out=[]
        while(nodes):
            ns=nodes.copy()
            ot=[i.val for i in ns]
            out.insert(0,ot)

            nn=[]
            for i in ns:
                if i.left:
                    nn.append(i.left)
                if i.right:
                    nn.append(i.right)

            # nnl=[i.left for i in ns if i.left]
            # nnr=[i.right for i in ns if i.right]
            
            # nn=nnl+nnr

            nodes=nn

        return out